The present invention relates to an electron emitting cathode for emitting photo-electrons or secondary electrons, and this cathode is applied to a photomultiplier, for example.
A conventional cathode for photo-electron or secondary electron emission is, for example, one stated in the Official Gazette on Japanese Patent Laid-Open No. 6-119872 (registered patent No. 2651329) claimed by the present applicant.
FIG. 8 is a perspective view depicting an example of this conventional cathode for photo-electronemission. The cathode for photo-electron emission 25 is comprised of a thin film 25d, where compound particles of alkali metal and antimony (Sb) are deposited on a nickel (Ni) electrode substrate 25c covered with an aluminum (Al) layer 25b. In this conventional cathode for photo-electron emission, the diameter of a particle constituting the thin film 25d is about the diffusion wavelength of an excited electron, where an improvement of electron emission efficiency (quantum efficiency) has been intended.
In a cathode for photo-electron emission, quantum efficiency normally depends on wavelength. In the case of a device using a cathode for photo-electron emission for detecting light, electrons or other charged particles, such as a photomultiplier, high quantum efficiency is generally desired in the entire wavelength region of the detection target or in a particle energy region wider than this area. In many cases, high quantum efficiency (red sensitivity) for wavelengths of about 600 to 800 nm of red to near infra-red is demanded. Whereas in the above mentioned conventional cathode for photo-electron emission, sensitivity in the wavelength region of about 600 to 800 nm is relatively low, and further improvement of red sensitivity has been desired.
With the foregoing in view, it is an object of the present invention to provide a cathode for photo-electron or secondary electron emission where quantum efficiency, particularly red sensitivity, can be improved, and the spectral sensitivity (photo-electron emission characteristic) or the secondary electron emission characteristic is improved. It is another object of the present invention to provide a photomultiplier and electronic multiplier having an excellent photo-electron emission characteristic or secondary electron emission characteristic.
To solve the above problems, the present inventors have eagerly researched the crystal particle growth process and film characteristics of a conventional cathode for photo-electron emission, where a thin film comprised of crystal particles of alkali antimony compound semiconductor is formed on an Al deposited substrate, and as a result the following was found out.
FIG. 9 is an enlarged cross-sectional view depicting the structure of the cathode for photo-electron emission 25 in FIG. 8 in cross-section IXxe2x80x94IX. As FIG. 9 shows, strain stress, which is generated by the differences of the thermal expansion coefficient and lattice constant with the Ni electrode substrate 25c, is applied to the particles 20 contacting the Al film 25b, and crystal defects 26 are generated as a result of relaxing this stress.
Here in the cathode for photo-electron emission 25, the cross-section of the particles 20 at the contact area tends to flatten due to the wettability between the Al film 25b and the particles 20. If this occurs, the strain stress applied to the particles 20 is increased, and the density of crystal defects 26 (defect density) increases. Also inside the particles which grow on the crystal particles 20 at the contact area, defects originating from a line defect or plane defect of the particles below are sequentially generated, and the defect density of an entire crystal particle 20 increases. In a crystal defect 26, the probability that an electron, excited by light or electrons, will recombine with a hole increases, and excited electrons which reach the surface of the crystal grain 20 decreases, which drops the quantum efficiency. The present inventors further promoted research based on these findings, and completed the present invention.
The present cathode for photo-electron or secondary electron emission is a cathode for photo-electron or secondary electron emission, comprising a thin film made of a material for emitting photo-electrons by the entry of light or for emitting secondary electrons by the entry of electrons, preferably a compound of at least one type of alkali metal and antimony metal, is formed on a substrate, characterized in that an intermediate layer comprised of carbon is disposed between the thin film and the substrate.
In the case of a cathode for photo-electron or secondary electron emission configured like this, a thin film, as a photo-electron emitting face, or a secondary electron emitting face for emitting photo-electrons or secondary electrons, is formed on the intermediate layer comprised of carbon disposed on the substrate. If this thin film is formed by depositing particles comprised of the above mentioned material for emitting photo-electrons or secondary electrons, photo-electrons or secondary electrons are emitted from the particles when light or primary electrons enter the particles constituting the thin film. Here it was found out that the deposited particles contacting the intermediate layer do not flatten, but maintain an elliptical shape, unlike prior art. The mechanism for such an interaction is not sufficiently understood, but it was confirmed that strain stress to be applied to the particles during particle growth is decreased, and internal crystal defects decrease.
Since the defect density of particles at the contact section with the intermediate layer decreases, defects which extend the crystal defects of the particles below are decreased in the grains which grow on the particles at the contact section. Therefore the crystal defect density in the particles of the entire thin film decreases the recombining probability of electrons excited by light or electrons and holes decreases remarkably, and the decrease of excited electrons reaching the surface of particles is controlled. Also even if the thin film is not formed by deposition of individually separated independent particles or particulate matter, and has such a deposition structure where many particles are connected together, or are in a state close to the film, crystal defects can be decreased since the above mentioned strain stress decreases.
It is preferable that the intermediate layer contains carbon nano-tubes. On the surface of the intermediate layer where particles of carbon nano-tubes are layered, micro-bumps in a nano-meter order are created. Such a shape and material characteristic drops wettability on the surface of the intermediate layer with respect to the particles constituting the thin film, which further controls the particles constituting the thin film to be deposited in a flattened state (such an interaction is not limited to this). As a result, the defect density in particles is further decreased, where the recombining probability of electrons and holes is further dropped, and the decrease of excited electrons which reaches the particle surface is further controlled.
It is also preferable that the thin film is activated by alkali metal, or that the thin film is comprised of a compound (alkali antimony compound) which consists of at least one type of alkali metal and antimony (Sb), particularly bi-alkali or a multi-alkali antimony compound.
The present photomultiplier or electronic multiplier has the above mentioned cathode for photo-electron emission or a cathode for secondary electron emission respectively. In other words, the present photo multiplier is characterized in that the above mentioned cathode for photo-electron emission is installed inside a vacuum container which has a light entrance window, where photo-electrons, which are emitted from the cathode for photo-electron emission by the incident light, are multiplied inside the vacuum container. The secondary electron multiplier is characterized in that the cathode for secondary electron emission of the present invention is installed in stages inside the container, where electrons to be detected enter the cathode for secondary electron emission at the first stage, and the secondary electrons emitted by the cathode for secondary electron emission at the last stage enter the anode. The xe2x80x9cvacuum containerxe2x80x9d is a sealed container where pressure inside the container is reduced to be roughly a vacuum.